Nottingham City Council – Highcross Court Tower block deconstruction

Connell Brothers were appointed as demolition contractors for the demolition of Highcross Court flats in Nottingham.

The building is a 15 storey high residential accommodation block in close proximity to residential housing and retail premises. The flats were constructed using large panel precast units to form the walls and precast planks to form the floors. The central core staircase and lift shafts were thought to be cast in-situ concrete, but upon further investigation, was found to be constructed from larger precast concrete panels. One side of the building incorporates a basement area and a sub-station was located at one end on the ground floor.

There was a limited amount of information available at tender stage. Some site investigations had been carried out to determine the panel thickness and construction, and pull out tests had been carried out to the external structure.

In order to develop a safe system of work, Connell Brothers carried out a series on on-site investigations to determine the location of the panel joints and the construction of the floors. This enabled the weights of each panel to be confirmed and a suitable tower crane sourced. A fully hard wired fire alarm system was also installed prior to works commencing.

The HSE Principal Inspector, Mr Kevin Wilson, was involved in the early stages of the development of this safe system of work.

Following the site investigation a propping scheme was developed to allow the use of 1 ½ and 3 tonne mini-excavators on the floors. The excavators were restricted to specific areas to allow for the removal of the bigger core panels whilst reducing the number of props required on each floor. The vertical panels, both internal and external, were also propped using push/pull props tied through the floors.

A tower crane was determined to be the most practical and cost effective method for the deconstruction works and external investigations were carried out to enable the tower crane base to be designed.

During these works the internal lift shaft was prepared for use as a drop shaft by removing the cars and rails and fully propping the base of the lift shaft which was above the basement. An opening was formed in the front of the building to enable a skid steer loader to remove the debris as soft stripping works were carried out.

A full height system scaffold was erected around the building. This was tied in a set pattern with the top two lifts untied and cantilevered above the two lifts below which were fully tied. This enabled the exterior panel to be removed at each floor level without the need to adapt the scaffold.

Once all the systems were in place soft stripping was carried out and the props installed to the top two levels. Acrow props were used to provide support to the floors and push/pull props to provide lateral support to the wall panels, both internally and externally. The initial scheme developed assumed the existing ties were insufficient to support the panels and, as the works progressed, the propping system was modified as panel ties were found to be intact.

Demolition commenced with the removal of the roof top communications mast and control cabin followed by the removal of the parapet wall panels.

During the initial removal of the parapet wall panels the mastic putty used to seal the joints was removed and tested. This was found to contain asbestos and the removal sequence for the entire project was modified to allow the mastic to be removed at each floor level, resulting in clean concrete panels being lowered to ground level for processing.

The wall panels were all single storey (2.5 m high) but investigation of the core determined that these panels were 2 storey high (5.0 m) and therefore of a much greater weight. The position chosen for the tower crane enabled these panels to be removed as single units, therefore eliminating the need to separate panels into smaller sections. GASS props and larger push/pull props were required in the core area to accommodate this different type of construction.

Demolition continued on a floor by floor basis by removing each floor to one side of the core at a time. This sequence enabled the scaffold to be struck to the non-working side whilst allowing demolition to continue to the other side. The tower crane then lowered the excavators to the floor below on the opposite side and the entire sequence repeated. Once two floors were removed the core could be demolished.

As demolition progressed the props were removed and reinstalled at the lower levels to ensure that two floors were always propped, not only to the working floor but also to the next available floor.

Connell Brothers Site Supervisor, as the Site Temporary Works Co-Ordinator, carried out daily checks to ensure the props were correctly installed.

Each floor and wall panel was lifted from the building and placed in a processing area where is was pulverised to remove the reinforcement and the resultant clean hardcore was removed from site to a recycling facility.

Demolition contimued until all the precast wall panels were removed (from the first floor upwards). The first floor slab was also removed prior to the tower crane being removed from the site. The remaining demolition works were carried out by ground based excavators. The remaining walls and the ground floor slab were removed and the foundations to a depth of 1 m below ground level. This left a basement to one half of the building. This was punctured through the base and backfilled with site won material. The entire footprint of the building was then brought up to ground level with site won material to provide a hazard free site for the following contractors.

Throughout the works Connell Brothers enjoyed a close working relationship with both the Client and the HSE. By carefully planning of the works, disruption to surrounding residents and retail premises was kept to a minimum.